diff --git a/README.md b/README.md
index c71afd4..e78c55a 100644
--- a/README.md
+++ b/README.md
@@ -25,6 +25,8 @@ Please refer to the README in each directory for more details.
   * /mozc-yunomi for firmware, schematics, and PCB layout.
 * [Gboard bar version](https://g.co/____), [blog post](https://japan.googleblog.com/2022/10/gboard-2022.html) (Oct 1, 2022)
   * /mozc-bar for firmware, schematics, and PCB layout.
+* [Gboard CAPS version](https://g.co/CAPS), [blog post](https://japan.googleblog.com/2023/10/caps.html) (Oct 1, 2023)
+  * /mozc-caps for firmware and hardware design.
 
 
 # Background stories (in Japanese)
diff --git a/mozc-caps/README.md b/mozc-caps/README.md
new file mode 100644
index 0000000..6b6a998
--- /dev/null
+++ b/mozc-caps/README.md
@@ -0,0 +1,161 @@
+# Gboard CAPS version
+
+This directory contains the firmware and hardware design for Gboard CAPS
+version, which was released on **Oct 1, 2023**.
+
+This is not an officially supported Google product.
+
+![Gboard CAPS version](./images/header.jpg)
+
+## Contents
+
+The directory structure is as follows:
+
+- hardware/ : STL files for mechanical parts.
+- firmware/ : Arduino sketch.
+
+## Building Gboard CAPS Version
+
+### Parts
+
+- M5StickC Plus
+- SS-10GL13 micro switch
+- [INC-100 bump cap insert](https://www.midori-helmet.jp/pickup/inc-100/)
+- [Grove connector cable](https://akizukidenshi.com/catalog/g/gC-16938/)
+- M3x10mm screw x 8
+- M2x8mm screw x 2
+- rubber band x 4
+- hot glue
+- heat shrink tubing
+- 160mm x 160mm x T2.5mm MDF board (or substitute a 3D printed part; see next
+  section)
+
+### Hardware
+
+#### Step 1: Prepare required parts
+
+3D printed parts:
+
+- [bump cap insert mount](./hardware/bump_cap_insert_mount.stl) x 2
+  \
+  ![bump cap insert mount](./images/bump_cap_insert_mount.png)
+- [sliding plate](./hardware/sliding_plate.stl) x 2
+  \
+  ![sliding plate](./images/sliding_plate.png)
+- [top plate angle bracket mount](./hardware/top_plate_angle_bracket_mount.stl)
+  x 2
+  \
+  ![top plate angle bracket mount](./images/top_plate_angle_bracket_mount.png)
+- [switch mount](./hardware/switch_mount.stl) x 1
+  \
+  ![switch mount](./images/switch_mount.png)
+
+Other parts:
+
+- top plate: made of 160mm x 160mm x T2.5mm MDF board
+  ![top plate](./images/top_board.png) \
+  Alternatively, you can 3D print this
+  part ([top_plate.stl](./hardware/top_plate.stl))
+
+#### Step 2: Wiring
+
+Connect the NO and the C terminals of the micro switch to the black and yellow
+wires of the Grove cable, in any order. (These will be connected to port G33 and
+GND of M5StickC Plus.)
+\
+![switch with wire harness](./images/switch_harness.jpg)
+
+#### Step 3: Build
+
+1. Put 2 rubber bands across the two latches on the sliding plates.
+   \
+   ![a](./images/attach_rubber_band.jpg)
+
+1. Attach one of the top plate angle bracket mounts to one of the sliding plates
+   and fasten using M3 screws. Repeat for the other set.
+   \
+   ![outer_rail to top_plate_holder](./images/assembly_1.png)
+
+1. Fasten the angle bracket mount and sliding plate assemblies to the top plate
+   using M3 screws \
+   ![top_plate_holder to top_plate 1](./images/assembly_2.png)
+   \
+   ![top_plate_holder to top_plate 2](./images/assembly_3.png)
+
+1. Screw the micro switch to the switch mount.
+   \
+   ![micro switch screwed on to a mount](./images/switch_with_mount.jpg)
+
+1. Glue the switch mount to one of the sliding plates. Check that the switch can
+   be pressed by testing using one of the bump cap insert mounts.
+   \
+   ![a](./images/switch_pressed.jpg)
+
+1. Attach the mounts to the left and right side of the cap, and adhere using hot
+   glue. \
+   ![a](./images/mount_inner_rail.jpg)
+
+1. Combine the two parts by attaching the sliding plates to the rails on the
+   bump cap insert mounts.
+
+1. Attach M5StickC Plus and connect the other side of the Grove connector that
+   is attached to the micro switch.
+
+### Firmware
+
+#### Step 1: Environment setup
+
+- Arduino IDE
+- Libraries
+  - [Madgwick library](https://github.com/arduino-libraries/MadgwickAHRS)
+    - You can install from Arduino IDE's Library Manager.
+  - [ESP32 BLE Keyboard library](https://github.com/T-vK/ESP32-BLE-Keyboard)
+    - You can download zip file from github and install from Sketch -> Include
+      Library -> Add .ZIP Library menu in Arduino IDE.
+  - [IMU Library](https://github.com/yamaguchi-am/m5stack-examples)
+    - Download
+      [imu.h](https://github.com/yamaguchi-am/m5stack-examples/blob/main/imu/imu.h)
+      and
+      [imu.cpp](https://github.com/yamaguchi-am/m5stack-examples/blob/main/imu/imu.cpp)
+      from this repository and put them into firmware/ directory.
+
+#### Step 2: Compile and upload
+
+- Compile and upload the mozc-caps.ino firmware to the M5StickC Plus using
+  Arduino IDE.
+
+#### Step 3: Calibration
+
+- IMU (gyro sensor) calibration
+  - After powering on the M5StickC Plus, the firmware will run calibration for
+    the IMU.
+  - While the display shows "calibrating" message, keep the device still.
+  - You can also trigger the calibration process by long-pressing Button A.
+- Setting the origin direction
+  - When Button A is pressed, the origin direction is set to the current
+    orientation of the device.
+
+#### Step 4: Connect
+
+Connect the keyboard via Bluetooth to a PC or other device that has Google
+Japanese Input/Gboard installed.
+
+Note: This keyboard keeps sending keycodes even before you press the key. It is
+for showing a preview of the selected character to be input on the connected
+device.
+
+## License
+
+```
+Licensed under the Apache License, Version 2.0 (the "License"); you may
+not use this file except in compliance with the License. You may obtain
+a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+```
diff --git a/mozc-caps/firmware/mozc-caps/.gitignore b/mozc-caps/firmware/mozc-caps/.gitignore
new file mode 100644
index 0000000..6713f3f
--- /dev/null
+++ b/mozc-caps/firmware/mozc-caps/.gitignore
@@ -0,0 +1,5 @@
+imu.cpp
+imu.h
+debug.cfg
+debug_custom.json
+esp32.svd
diff --git a/mozc-caps/firmware/mozc-caps/mozc-caps.ino b/mozc-caps/firmware/mozc-caps/mozc-caps.ino
new file mode 100644
index 0000000..18f10ce
--- /dev/null
+++ b/mozc-caps/firmware/mozc-caps/mozc-caps.ino
@@ -0,0 +1,172 @@
+#include <BleKeyboard.h>
+#include <M5StickCPlus.h>
+
+#include "imu.h"
+
+// Comment out the line below to use English alphabet mode.
+#define JAPANESE_
+
+#ifdef JAPANESE_
+// Japanese (hiragana) mode. Must be used with romaji input mode.
+
+// clang-format off
+const char* characters[] = {
+  "a", "i", "u", "e", "o",
+  "ka", "ki", "ku", "ke", "ko",
+  "ga", "gi", "gu", "ge", "go",
+  "sa", "si", "su", "se", "so",
+  "za", "zi", "zu", "ze", "zo",
+  "ta", "ti", "tu", "te", "to",
+  "da", "di", "du", "de", "do",
+  "na", "ni", "nu", "ne", "no",
+  "ha", "hi", "hu", "he", "ho",
+  "ba", "bi", "bu", "be", "bo",
+  "pa", "pi", "pu", "pe", "po",
+  "ma", "mi", "mu", "me", "mo",
+  "ya", "yu", "yo",
+  "ra", "ri", "ru", "re", "ro",
+  "wa", "wo", "nn",
+  "lya", "lyu", "lyo", "ltu",
+  "-",
+};
+// clang-format on
+
+#else
+
+// English alphabet mode.
+
+const char* characters[] = {"a", "b", "c", "d", "e", "f", "g", "h", "i",
+                            "j", "k", "l", "m", "n", "o", "p", "q", "r",
+                            "s", "t", "u", "v", "w", "x", "y", "z", " "};
+
+#endif
+
+// A class that handles character input, interactively showing
+// unsettled characater on the target device.
+class Compositor {
+ public:
+  Compositor() : last_character_unsettled_(false) {}
+  // TODO: add commit() which commits the Japanese composition by Enter key.
+  void enter();
+  void select(const char* str);
+
+ private:
+  bool last_character_unsettled_;
+};
+
+const float kSamplingFrequency = 100.;
+const float kIntervalMicros = 1e6 / kSamplingFrequency;
+const uint8_t PUSH_SW = G33;
+
+BleKeyboard bleKeyboard("mozc-caps");
+M5StackIMUManager imu;
+Compositor compositor;
+long last_time_micros;
+float yaw_origin = 180;
+int current_key = 0;
+int last_key = 0;
+
+void Compositor::enter() {
+  if (!last_character_unsettled_) {
+    return;
+  }
+  last_character_unsettled_ = false;
+}
+
+void Compositor::select(const char* str) {
+  if (last_character_unsettled_) {
+    // It is assumed there is only one unsettled character,
+    // as the IME should be set to the romaji input mode.
+    bleKeyboard.write('\b');
+  }
+  for (const char* p = str; *p; p++) {
+    bleKeyboard.write(*p);
+  }
+  last_character_unsettled_ = true;
+}
+
+void setup() {
+  M5.begin();
+  M5.Lcd.setRotation(3);
+  M5.Lcd.fillScreen(BLACK);
+  imu.Init();
+  last_time_micros = micros();
+  pinMode(PUSH_SW, INPUT_PULLUP);
+  bleKeyboard.begin();
+}
+
+void loop() {
+  float gyro_x, gyro_y, gyro_z, acc_x, acc_y, acc_z;
+  M5.IMU.getGyroData(&gyro_x, &gyro_y, &gyro_z);
+  M5.IMU.getAccelData(&acc_x, &acc_y, &acc_z);
+  // Convert coordinate system to use the rotation around the Y axis as "yaw"
+  // angle.
+  imu.Update(kSamplingFrequency, gyro_z, gyro_y, -gyro_x, acc_z, acc_y, -acc_x);
+  float pitch, roll, yaw;
+  imu.GetAHRSData(&pitch, &roll, &yaw);
+
+  M5.update();
+  if (M5.BtnA.wasPressed()) {
+    yaw_origin = yaw;
+  }
+  if (M5.BtnA.pressedFor(1000)) {
+    imu.Init();
+  }
+
+  float relative_angle = yaw - yaw_origin;
+  if (relative_angle > 180) {
+    relative_angle -= 360;
+  } else if (relative_angle < -180) {
+    relative_angle += 360;
+  }
+
+  bool key_pressed = false;
+  if (digitalRead(PUSH_SW) == LOW) {
+    key_pressed = true;
+  }
+
+  // The range of rotation angle [deg] from the center, used for selecting
+  // characters.
+  const float kAngleRange = 80;
+  int num_keys = sizeof(characters) / sizeof(characters[0]);
+  float position = (relative_angle + kAngleRange) / kAngleRange / 2 * num_keys;
+  int key_index = static_cast<int>(position + 0.5);
+  if (key_index < 0) key_index = 0;
+  if (key_index > num_keys - 1) key_index = num_keys - 1;
+  float residual = position - key_index;
+  if (residual < -0.5) residual += 1;
+  if (residual > 0.5) residual -= 1;
+  if (residual < 0) {
+    residual = -residual;
+  }
+  if (residual < 0.4 && !key_pressed) {
+    current_key = key_index;
+    if (current_key != last_key) {
+      compositor.select(characters[current_key]);
+    }
+    last_key = current_key;
+  }
+  if (key_pressed) {
+    compositor.enter();
+  }
+
+  M5.Lcd.setTextSize(1);
+  M5.Lcd.setTextColor(OLIVE, 0x000000);
+  M5.Lcd.setCursor(30, 110);
+  if (imu.Ready()) {
+    M5.Lcd.printf("                           ");
+  } else {
+    M5.Lcd.printf("calibrating gyro sensors...");
+  }
+
+  M5.Lcd.setTextSize(7);
+  M5.Lcd.setCursor(0, 24);
+  M5.Lcd.setTextColor(key_pressed ? YELLOW : DARKGREEN, 0x000000);
+  M5.Lcd.printf(" %3s ", characters[current_key]);
+
+  long next_time_micros = last_time_micros + kIntervalMicros;
+  while (micros() < next_time_micros) {
+    delay(1);
+  }
+  last_time_micros = next_time_micros;
+}
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